Process for treating elastomeric fibers



United States Patent Office 3,536,803 Patented Oct. 27, 1970 3,536,803 PROCESS FOR TREATING ELASTOMERIC FIBERS Martin Eden Epstein, Warren Township, Somerset County, Arnold Joseph Rosenthal, Morris, and Joseph Germano Santangelo, Union, N.J., assignors to Celanese Corporation, a corporation of Delaware No Drawing. Filed May 26, 1964, Ser. No. 370,371 Int. Cl. D01d 5/12 US. Cl. 264-210 15 Claims ABSTRACT OF THE DISCLOSURE Elastomeric fibers comprised of alternating hard and soft segments, i.e. polyester-urethane copolymers, are improved in properties, particularly permanent set, when stretched at least 150% of initial length, relaxed and subjected to a heat treatment while at a constant length between about 75 C. and the softening point of the fiber.

Our invention relates to process for producing an improved elastomeric fiber and more particularly relates to a process for producing such an improved fiber by stretching, relaxing and heat treating the fiber.

We have found that an improved elastomeric fiber can be obtained by stretching a fiber of a segmented elastomeric copolymer at least about 150% of its initial length, then relaxing the stretched fiber so that it returns to a length less than its stretched length and finally heating the stretched and relaxed fiber at constant length at a temperature above about 75 C. and below the softening point of the fiber. In practicing the process of our invention it is essential that all three steps of the process be employed, since neither the stretching and relaxing steps alone without the heat treating nor the heat treating Without the prior stretching and relaxing will produce a fiber with all the desirable characteristics attained in accordance with our invention. Moreover, any desirable characteristics which might possibly be obtained as a result of employing one of the steps separately are not necessarily permanent. For example, certain desirable characteristics can be imparted to the fiber by stretche ing, but such characteristics are not stable when the fiber is subjected to exposure to boiling water as is encountered in conventional textile operations, such as scouring and/or dyeing.

In the stretching step of our process, a stretch of at least about 150% and usually in the range of from 200% to 500% over the initial length of the fiber, i.e., the length of the fiber in its unstretched state prior to any treatment in accordance with our invention, is operable to impart desirable characteristics to the fiber when employed in conjunction with the remainder of our process. This initial length can be the length of a somewhat elongated fiber resulting from draw down or draft coincident with spinning. Preferably, however, the amount of stretch employed is from about 250% to about 400%. The period of time during which the fiber is maintained in the state of stretch is not critical to the advantageous employment of our process and can vary from periods of less than a second up to periods of several minutes. The fiber, however, must be maintained at a temperature below its softening point. Normally, the stretching can be conducted at about room temperature.

The relaxing step of our invention can be conducted at about the same temperature as the stretching step and generally is conducted at about room temperature. The period of time during which the fiber is maintained in the relaxed state is not critical.

In the heating step of our process, a temperature above about C. has been found to be elfective, and generally, any temperature up to the softening point of the fiber is effective. Once the softening point of the fiber has been attained there is a tendency for some of the desirable characteristics which have been imparted to be lost. A temperature in the range from about to about C. is adequate to develop desirable characteristics in elastomeric fibers produced from segmented elastomeric copolymers, with particularly advantageous results being obtained with temperatures ranging from about 100-120 C. The length of time a fiber is subjected to heating in accordance with our invention can vary from a few minutes up to several hours. Usually, the general rule of employing a longer time at a lower temperature and a shorter time at a higher temperature is applicable.

The term segmented elastomeric copolymer as used throughout this specification and in the claims is meant to describe elastomeric copolymers comprised of two principle types of segments which are chemically connected and alternate in the chemical chain. One segment, preferably essentially amorphous, is derived from low melting soft polymers such as, for example, an aliphatic ester polymer, an ether polymer, a hydrocarbon polymer, and the like. These soft polymers are characterized by weak interchain attractive forces. The other segment, derived from a hard high melting polymer such as, for example, a urea polymer, a urethane polymer, amide polymer, and the like.

In particular, the soft segments of these elastomers are derived from low melting polymers having a melting point below about 60 C., having a molecular weight of from about 250 to about 5,000 and containing terminal radicals possessing active hydrogen atoms. The hard, high melting segments are derived from linear hard polymers having a melting point above about 200 C. in their fiber forming molecular weight range, i.eabove about 5,000. The soft segments, as present in the elastomer appear as radicals of the initial polymer from which the terminal active hydrogens have been removed. Generally, the hard segments comprise from about 10% to about 40% by weight of the segmented copolymer and may be defined as comprising at least one repeating unit of the linear crystalline polymer from which they are derived.

The preparation of these segmented elastomeric copolymers is well known in the art and is described, for instance, in US. Pat. Nos. 2,625,535; 2,813,776; 2,871,218; 2,953,839; 2,957,852.; 2,962,470 and Re. 24,691.

The polymeric structure of some of these elastomers can be represented by the formula:

wherein H and S denote the hard and soft segments, respectively; and wherein H is further represented by wherein Q is a divalent radical derived by reaction of an organic diisocyanate with active-hydrogen-containing functional groups; G is the residue resulting from the removal of active hydrogen from the terminal functional groups of a low molecular weight bifunctional molecule having a molecular weight less than 500; x is an integer or zero; b is an integer greater than zero; S is the residue resulting from the removal of the active hydrogen from the terminal functional groups of a polymer melting below 60 C., having a molecular weight of from about 250 to about 5,000. Terminal functional groups possessing active hydrogen can be, for example, OH, NH SH, COOH, CONH =NH, CSNH SO NH and SO OH. The hard segment in a repeating unit of a linear polymer may be further defined as having a melting point above about 200 C. in its fiber forming molecular weight range.

Generally, these synthetic elastomers are copolymer formulations based on low molecular weight aliphatic polyesters or polyethers having terminal hydroxyl groups which are capable of further reaction with diisocyanates. This latter reaction can be used to couple the lower molecular weight polyester or polyether via urethane links or the diisocyanate can be used in excess so that it becomes a terminal group. In this latter case, the macro diisocyanates formed can be coupled by means of other reagents such as water, diols, amino alcohols and diamines with the subsequent formation of the high polymer. These elastomeric products are also known as block copolymers.

A variety of organic diisocyanates may be used to prepare the elastomeric copolymers suitable for employment in our invention. Illustrative examples of these diisocyanates are: trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, decamethylene diisocyanate, cyclopentylene-1,3-diisocyanate, 1,4 diisocyanate cyclohexane, p-phenylene diisocyanate, m-phenylene diisocyanate, the tolylene diisocyanate, the naphthalene diisocyanates, 4,4'-diphenyl propane diisocyanate, 4,4-diphenylmethane diisocyanate.

Illustrative of the types of elastomeric copolymers suitable for employment in our invention are isocyanate modified polyesters such as those in US. Pat. 2,755,266 wherein linear polyesters prepared from polycarboxylic acids and polyhydric alcohols are reacted with an excess of a diisocyanate over the terminal hydroxyl groups of the polyesters to form diisocyanate modified polyesters containing terminal isocyanate groups which are then further reacted with a bifunctional cross-linking agent. Polyesterurethane copolymers which are substantially free of cross-links, such as those described in US. Pat. 2,871,218 wherein a critical ratio of an essentially linear hydroxyl terminated polyester prepared from a saturated aliphatic glycol having terminal hydroxyl groups and a dicarboxylic acid or its anhydride, and a diphenyl diisocyanate are reacted in the presence of a saturated aliphatic free glycol having terminal hydroxyl groups so that no unreacted isocyanate and hydroxyl groups remain, can also be employed. Broadly such a copolymer is obtained by reacting one mole of polyester with from 1.1 to 3.1 moles of a diphenyl diisocyanate in the presence of from about 0.1 to 2.1 moles of free glycol. Another type of elastomeric copolymer which can be used in our invention is the type described in US. Pat. 2,957,852. An elastomer of this type can be prepared by providing polyether glycol with isocyanate ends by reaction with a diisocyanate. This capped prepolymer can then be reacted with a chainextending agent such as a hydrazine which provides a final polymer having repeating units containing hydrazine resins linked through carbonyl groups.

In another aspect, our invention comprises further adjusting such characteristics and properties of an elastomeric fiber as elongation, tenacity, and permanent set by a judicious blending of varying proportions of a comparatively flexible and a comparatively stiff segmented, elastomeric copolymer. The flexibility or stiffness of two elastomeric copolymers is determined by the ratio of the amount of polyurethane to polyester or polyether in the block copolymer with, of course, the more flexible copolymers being that which has a greater proportion of the soft polymer.

In this aspect of our invention, both the stiff and the flexible copolymers are of the type described above. For example, both the stiff and the flexible copolymers can be synthesized from substantially the same starting material or at least starting materials of the same type. Thus a soft or flexible copolymer of the type described in US. Pat. 2,871,218 can be produced by employing as starting materials larger proportions of the linear polyester and the aliphatic glycol thereby producing a segmented copolymer having a greater number of amorphous or soft blocks. Similarly, a higher molecular weight amorphous polymer employed as a starting material will provide a copolymer having longer soft blocks. To obtain a comparatively stiff copolymer a larger proportion of the diisocyanate, the linear crystalline component, can be employed thereby producing a segmented copolymer having a larger number of rigid blocks or a copolymer having longer rigid blocks depending upon the relative proportions of the other ingredients or upon the molecular weight of the polyester. Thus, it will be seen that the stiffness or flexibility of the resulting copolymer can be affected by only very small variations in the molar ratios of the ingredients and/or employing a polyester of a higher or lower molecular weight.

The advantageous results obtained through this aspect of our invention are extremely surprising and unexpected inasmuch as blends including up to of flexible or soft copolymers, which alone cannot withstand the severity of the steps of our process, particularly the heating, can be employed to yield extremely desirable fibers. Usually, the flexible copolymers alone are unstable as fibers at temperatures above about C. and have been disintegrated on the bobbin at temperatures of about C. The stiff copolymers alone are generally stable as fibers up to temperatures of at least 200 C.

Furthermore, the characteristics possessed by a fiber spun from a blend are superior to those of a fiber spun from a single copolymer of comparable chemical composition. More specifically, a blend of a stiff copolymer obtained by reacting given quantities of certain reactants and a flexible copolymer obtained by reacting other given quantities of certain reactants in the manner described above yields a fiber having more desirable characteristics than a fiber produced from a single copolymer obtained by reacting together the same quantities of all of the reactants employed to produce the stiff and the flexible copolymers.

In order to illustrate further our invention, reference is made to the following examples. In each of these examples permanent set was determined by the same method unless otherwise indicated. Briefly, all samples were conditioned for 24 hours at 23 C. and 65% relative humidity prior to testing. Each sample was then marked, While in a taut (not stretched) state, to delineate a 3.00 inch test length. The sample was then clamped between jaws set 5 inches apart so that the 3.00 inch test length was midway between the jaws. The jaws were then moved apart so that the sample was extended until the distance between the marks (3.00 inches before extension) was 12.00 inches for a 300% extension. After maintaining the sample in its extended state for 2 hours, the sample was released and allowed to recover unrestrained for 30 minutes. The sample was then made taut (not stretched) and the length between the marks was measured. The increase in the length between the marks over the original 3.00 inches expressed as a percent of the original 3.00 inch length is the permanent set.

EXAMPLE I A comparatively soft or flexible polyester-urethane copolymer of the type described in US. Pat. 2,871,218 and obtained by reacting hydroxyl poly (tetramethylene adipate) (molecular weight about 850, hydroxyl nurnber=130.4, acid number=0.89) butanediol-1,4 and diphenyl methane-p,p'-diisocyanate in a molar ratio of about 1.0:0.3:1.3, respectively, was employed in this example. A 30% solution of this copolymer in acetone solvent was employed to dry spin an elastomeric fiber.

The apparatus used was of the type traditionally employed in the art and essentially included a spinnerette at the upper end of a spinning column and a godet roll at the bottom of the column. The filaments were passed through the column where the solvent was substantially evaporated therefrom by contact with hot air introduced at a temperature of 150 C. and was then passed about the godet roll moving at a velocity of meters/ minute.

After leaving the godet roll, the fiber was subjected to a drawing operation at room temperature, in which the first draw roll was travelling at a speed of 20 meters per minute and the second draw roll was travelling at a speed of about 100 meters per minute thereby effecting a stretch of 400%. After being stretched by means of the drawing operation, the fiber, at room temperature, was relaxed by removal of all but a minimum amount of tension necessary to handling. The fiber was then wound on a bobbin. Physical testing of the fiber as spun indicated a tenacity of 0.64 grams per denier, elongation of 450% and a permanent set of 3.0% (4 hours at 300% strain). Subsequently, the fiber, no longer on the bobbin, was subjected to boil off treatment in steam at 100 C. for one half an hour. This resulted in a boil-ofl shrinkage of and an increase in permanent set from the original 3.0% up to Thus, it can be seen that desirable characteristics, particularly the permanent set of the stretched fiber, in the absence of heat treating were completely destroyed by the steam boil off.

EXAMPLE II In this example a comparatively stiif polyester-urethane copolymer of the type described in US. Pat. 2,871,218 was employed. The copolymer was obtained by reacting hydroxyl poly (tetramethylene adipate) (molecular weight stretched fiber. The gain in strength, however, was lost when the stretched fiber was subjected to boil-off as shown by the decrease in tenacity from 1.62 to 0.67. Furthermore, subjecting the stretched fiber to boil-0E also produces an undesirably high permanentset, i.e., 56%.

EXAMPLE III In this example the same copolymer as used in Example II was employed. A 25% solution of the copolymer in 91/9 mixture of methylene chloride/methanol solvent was spun to produce an elastomeric fiber. In order to produce a complete basis of comparison, a portion of the uber was taken-up without being subjected to either stretching or heat treatment. Another portion of the fiber was subjected to a drawing operation at room temperature after leaving the spinning column in which the fiber was stretched 300%. After stretching, the fiber was relaxed at room temperature by removal of all but a minimum amount of tension necessary to handling. The fiber was maintained in this relaxed state While it returned to a length intermediate between initial and stretched length. A sample of both the stretched and unstretched fiber wound on a bobbin and maintained at constant length was subjected to heat treatment. Samples of both the stretched and unstretched fibers which were not heat treated were subjected to boil-ofi treatment in 100 C. steam for half an hour as were the heat-treated stretched and unstretched fibers. Samples of each of the variously treated fibers were then subjected to physiabout 1010, hydroxyl number=106.1), butanediol-1,4 and 30 cal testing. The results are given below in Table II.

TABLE IL-IMPROVEMENT OF FIBER PROPERTIES ON HEAT SETTING 1 One hour at 145 C.

diphenyl methane-p,p-diisocyanate in a molar ratio of about 1:1:2, respectively. A 30% solution of this copolymer in a methylene chloride-10% methanol solvent mixture was employed to dry spin an elastomeric fiber. The apparatus employed was the same as that used in Example I. A portion of the fiber was taken-up directly from the spinning column without stretching and wound on bobbins. A second portion of the fiber was subjected to a drawing operation at room temperature after leaving the spinning column to obtain a stretch of 350% (4.5 X). After being stretched by means of the drawing operation, the fiber, at room temperature, was relaxed by removal of all but a minimum amount of tension necessary to handling and was then wound on bobbins. Each of these portions of the fiber, the unstretched, and the stretched and relaxed, was again divided into two portions, one portion of each of the unstretched, and the stretched and relaxed fiber, was subjected to boil-off treatment in C. steam for half an hour. Samples of each of the four portions of the fiber were subjected to physical testing. The results are indicated below in Table I.

These data demonstrate the extremely advantageous characteristics possessed by fibers produced in accordance with our invention. A comparison of the characteristics of the stretched and relaxed fiber as spun with the characteristics of the stretched and relaxed fiber which has been subjected to boil-otI demonstrates the loss of desirable characteristics resulting from boiloff. In fact, the stretched, relaxed and boiled-off fiber possesses characteristics not greatly, if at all, superior to the unstretched fiber which has been subjected to boil-off. Examination of the characteristics, of the unstretched fiber which has been heat treated demonstrates that heat treating alone does not, of itself, produce a fiber having extremely desirable characteristics. Thus, for example, the heat-set, unstretched fiber possesses characteristics in many intances quite similar or only slightly improved over the stretched and unstretched fiber after boil-01f. Thus, while heat treatment alone may improve permanent set to a certain degree, the permanent set is still not at a desirable level. A comparison of the physical characteristics TABLE I.PROPERTIES OF STRETCHED AND UNS'IRETCHED FIBERS Permanent set, pereent A comparison of the characteristics of the as spun unstretched and the stretched and relaxed fibers demonstrates that the stretching increases the strength of the fiber as indicated by the increased tenacity of the attained by a fiber which has been subjected to stretching, relaxing and heat treatment and then boil-off with the characterisics of all the other fibers shown in Table II demonstrates the permanence of the desirable characteristics, particularly tenacity, stress at 300% elongation and permanent set.

EXAMPLE IV In this example fibers were dry spun from solutions of the copolymer employed in Example II and a blend of the copolymers of Examples I and II, and a comparison was made of the physical characteristics of the fibers obtained both before and after heat treating. It will be noticed in the data shown below that a satisfactory fiber is obtained from the blend of copolymers. This is particularly unexpected, first, due to the tremendous increase in permanent set obtained during boil-off of the copolymer of Example I, and second, due to the fact that previous attempts to heat treat the copolymers of Example I were completely ineffective since this copolymer has a tendency to decompose even under the mildest heat treating and actually disintegrates on the bobbin at a temperature of about 100 C. The proportions of copolymer blends employed, along with the composition of the spinning solution and the operating conditions are listed below in Table III. All of the stretched fibers were relaxed at room temperature by removal of all but a minimum of tension necessary to handling and the heat treating consisted of maintaining the fibers on a bobbin in a circulating air oven maintained at 110 C. for a period of two hours.

TABLE III Composition, Ex. I/Ex. II /100 25/75 Polymer I.V. 1. 10 0. 90/1. 10 Pigment, percent T102 5 5 Dope solids, weight percent 25 26 Top cabinet temp, 0. (air) 75-80 75. 80 Bottom cabinet temp, G. (air) 195-200 195-200 After stretch, percent 350 350 Final spinning speed, m./min 60 150 As Boiled- As Boiledspun ofi spun 01? Properties before heat treatin Denier 180 360 100 250 Tenacity, g./d 1. 6 0.7 l. 8 0. 7 Elongation, percent 280 550 250 590 Boiling water shrinkage, percent--. 50 60 Stress at 200% strain, g./d 0. 75 1.1 Stress at 300% strain, g./d 0. 14 0. 11 Permanent set, percent 26 21 Properties after heat treating Denier 180 200 I00 110 Tenacity, g./d 0. 8 0. 3 0. 8 Elongation, percent 410 4.40 4530 Boiling Water shrinkage, percent..-" 10 Stress at 300% strain, g./d 0. 34 0. 28 Permanent s et, percent 18 13 1 I.V.intrinsic viscosity.

The data of Table III not only demonstrate an additional example of the stretching, relaxing and heat treating of our invention, but also illustrate another aspect of our invention, i.e., blending a relatively soft or flexible copolymer with a relatively stiff copolymer to provide a satisfactory fiber. It will be noted that the addition of the copolymer from Example I to the copolymer from Example II results in slight alterations of the fibers characteristics, i.e., a higher elongation with a lower modulus (stress at 300% strain) and a lower permanent set.

Any departure from the above description which conforms to the present invention is intended to be included within the scope of the invention as defined by the following claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing an improved elastomeric fiber composed of a segmented, elastomeric copolymer represented by the formula:

wherein H denotes the hard segment and S denotes the soft segment; and wherein H has a melting point above about 200 C. in its fiber-forming molecular weight range and is further represented by wherein Q is a divalent radical derived by reaction of an organic diisocyanate with active-hydrogen containing functional groups; G is the residue resulting from the removal of active hydrogen from the terminal functional groups of a low molecular weight bifunctional molecule having a molecular weight less than 500; x is an integer from 0 to 1; b is an integer greater than zero; and, wherein S is the residue resulting from the removal of the active hydrogen from the terminal functional groups of a polymer melting below 60 C. having a molecular weight of from about 250 to about 5000, which comprises the steps of:

-(1) dry spinning said polymer;

(2) stretching said fiber at least about 150% of its initial length;

( 3) relaxing the stretched fiber to return it to a length less than the stretched length of the fiber; and

(4) heating the stretched and relaxed fiber at a constant length at a temperature above about 75 C. and below the softening point of the fiber.

2. The process of claim 1 wherein the fiber is stretched from about 200% to about 500% of the initial fiber length.

3. The process of claim 1 wherein the fiber is stretched from about 250% to 400% of the initial fiber length.

4. The process of claim 1 wherein the stretched and relaxed fiber is heated at a temperature from about to about C.

5. The process of claim 1 wherein the soft segment is derived from a material selected from the group consisting essentially of polyesters and polyethers and the hard segment is derived from a material selected from the group consisting of alkyl-, aryl-, alkaryland aralkyl-di isocyanates.

6. A process for producing an improved elastomeric fiber which comprises the steps of:

( 1) dry spinning a fiber of segmented elastomeric copolymer;

the elastomeric copolymer being comprised essen tially of soft and hard segments alternating in the copolymer chain in which the soft segment is derived from the group consisting of linear polyesters and linear polyethers melting below about 60 C. and having a molecular weight from about 250 to about 5000 and the hard segment is derived from a material selected from the group consisting of linear alkyl-, aryl-, alk aryland aralkyldiisocyanates;

(2) stretching the dry spun fiber from about 200% to about 500% of the initial fiber length;

(3) relaxing the stretched fiber to return said fiber to a length intermediate the initial length and the stretched length of the fiber; and

(4) heating the stretched and relaxed fiber at a constant length at a temperature from about 100 to about 150 C.

7. The process of claim 6 wherein the soft segment is derived from an essentially linear hydroxyl terminated polyester obtained by the reaction of a linear glycol hav ing from 4 to 8 carbon atoms and a dibasic aliphatic acid having from 4 to 10 carbon atoms, the polyester having a molecular weight from about 700 to 1100 and the hard segment is derived from a diphenyl diisocyanate having an isocyanate group on each phenyl nucleus and wherein the fiber is stretched from about 250% to about 350% of its initial length.

8. A process for producing an improved elastomeric fiber which comprises the steps of:

(1) dry spinning said fiber;

(2) stretching said fiber of a segmented elastomeric copolymer from about 250% to about 400% of the initial fiber length;

the elastomeric copolymer being comprised of soft and hard segments alternating in the copolymer chain in which the soft segment is derived from an essentially hydroxyl terminated poly (tetramethylene adipate) having a molecular weight of about 1000 and the hard segment is derived from diphenyl methane-p,p-diisocyanate;

(3) relaxing the stretched fiber to return said fiber to a length intermediate the initial length and the stretched length of the fiber; and

(4) heating the stretched and relaxed fiber at a constant length at a temperature from about 100 to about 9. A process for producing an improved elastomeric fiber which comprises the steps of:

( 1) dry spinning a fiber from a blend of fiber-forming segmented, elastomeric copolymer, each of which is represented by the formula:

wherein H denotes the hard segment and S denotes the soft segment; and wherein H has a melting point above about 200 C. in its fiber-forming molecular weight range and is further represented by Q Q 'x wherein Q is a divalent radical derived by reaction of an organic diisocyanate with active-hydrogen containing functional groups; G is the residue resulting from the removal of active hydrogen from the terminal functional groups of a low molecular weight bifunctional molecule having a molecular Weight less than 500; x is an integer from to 1; b is an integer greater than zero; and wherein S is the residue resulting from the removal of the active hydrogen from the terminal functional groups of a polymer melting below 60 C. having a molecular weight of from about 250 to about 5000,

(a) one of the copolymers being a flexible copolymer unstable as a fiber at temperatures above about 100 0., and

(b) another of the copolymers being a stilf copolymer stable as a fiber at temperatures up to at least about 150 C.;

(2) stretching the fiber at least about 150% of the initial fiber length;

(3) relaxing the stretched fiber to return said fiber to a length less than the stretched length of the fiber; and

(4) heating the stretched and relaxed fiber at a constant length at a temperature above about 75 C. and below the softening point of the fiber.

10. The process of claim 9 wherein the fiber is stretched from about 200% to about 500% of the initial length of the fiber.

11. The process of claim 9 wherein the fiber is stretched from about 250% to 400% of the initial length of the fiber.

12. The process of claim 9 wherein the stretched and 10 relaxed fiber is heated at a temperature from about to about C.

13. The process of claim 9 wherein the soft segment is derived from a material selected from the group consisting essentially of polyesters and polyethers and the hard segment is derived from a material selected from the group consisting of alkyl-, aryl-, alkaryland aralkyl-diisocyanates.

14. The process of claim 9 wherein the comparatively flexible copolymer comprises up to about 50% by weight of the total blend of the copolymers.

15. A process for producing an improved elastomeric fiber which comprises the steps of:

(1). dry spinning a fiber from a blend of fiber-forming segmented elastomeric copolymers which copolymers are comprised essentially of soft and hard segments alternating in the copolymer chains in which the soft segment isderived from the group consisting of linear polyesters and linear polyethers melting below about 60 C. and having a molecular weight from about 250 to about 5000 and the hard segment is derived from a material selected from the group consisting of linear crystalline alkyl-, aryl-, alkaryland aralkyldiisocyanates, which blend comprises:

(a) up to 50% by weight of a comparatively flexible copolymer unstable as a fiber at temperatures above about 100 C., and

(b) at least 50% by weight of a comparatively stifi copolymer stable as a fiber at temperatures up to at least about 150 C.;

(2) stretching the fiber from about 200% to about 500% of the initial length of the fiber;

(3) relaxing the stretched fiber to return it to a length intermediate the initial length and the stretched length of the fiber; and

(4) heating the stretched and relaxed fibers at a constant length at a temperature from about 100 to about 150 C.

References Cited UNITED STATES PATENTS 3,117,906 1/1964 Tanner 264-171 3,365,874 1/1968 Chidgey et al 264-168 X 3,377,308 4/1968 Oertel et al. 26032.6 3,387,448 6/1968 Lathem et al. 57l52 3,339,000 8/1967 Vance 264-184 3,402,236 9/1968 Goodwin.

2,962,470 11/1960 Jung 26045.4 3,047,909 8/ 1962 Boyer 1848 FOREIGN PATENTS 25,165 11/1963 Japan.

916,287 1/ 1963 Great Britain.

JULIUS FROME, Primary Examiner I. H. WOOD, Assistant Examiner U.S. Cl. X.R. 

